Control system



CONTROL SYSTEM Filed March 22, 1941 M. A; EDWARDS EI'AL @m m 6 w m E R3r w a 00 5 v ..o 4 .m .0 Q E fi- M Na a W #0 z a U I fi m muh U 1IIIIIIIIIIIIIIHMHIIRI IWQUMQ m 0 ab I lllwllllllllllllllllll W* w w m aw Get. 13, 1942.

' Martin A. Edwards, Francis Mohler by FM c:

Their" Attorngy.

Patented Oct. 13, 1942 CONTROL SYSTEM Martin A. Edwards, Scotia, andFrancis Mohler,

Schenectady, N. Y., assignors to General Electric Company, a corporationof New York Application March 22, 1941, Serial No. 384,118

11 Claims.

This invention relates to control systems, more particularly to systemsfor controlling the acceleration. running and deceleration of anelectric motor, and it has for an object the provision of an improved,simple, reliable and inexpensive control system of this character.

The invention also relates to a control system for matching the speedsof two or more motors. and a further object of the invention is theprovision of a highly accurate regulating control system of thischaracter.

In carrying the invention into effect in one form thereof, means areprovided for producing a control voltage proportional to the speed of afirst electric motor, and means are provided for producing a voltageproportional to the speed of a second electric motor. A dynamoelectric mlchine is provided with a control field winding which is connected to beresponsive to these voltages. and this dynamoelectric machine controlsthe speed of one of the motors so as to bring about and maintain apredetermined speed relationship between the two motors.

In a specific embodiment of the invention, a pilot generator driven bythe motor whose speed is to be used as the reference speed is providedfor producing the voltage which is proportional to this reference speed,and the control field winding of the dynamoelectric machine is energizedin accordance with the difference of the reference speed voltage and avoltage which is proportional to the speed of the controlled motor.

An added feature of the invention is the provision of a fine or verniercontrol. This feature embodies means such. for example. as tachometer Ienerators driven by the reference speed motor and the controlled motorfor producing control voltages proportional to the speeds of the motors.together with means responsive to the difference of these voltages formodifying the excitation of the pilot generator driven by the referencespeed motor.

The means for modifying the excitation of the pilot generator ispreferably a vacuum tube ampliiler connected to be nonconducting whenthe speed of the controlled motor is less than a predetermined valuerelative to the speed of the reference speed motor. and to becomeconducting when the speed of the controlled motor equals or exceeds thedesired value relative to the speed of the reference speed motor.

In illustrating the invention in one form thereof, it is shown asembodied in a control system for a flying shear. The operatingrequirements of a flying shear with respect to speed and ac- Iiii curacyare unusually rigid and for this reason the application of the inventionto a flying shear is well adapted to illustrate the novel and usefuloperating characteris ics of the invention.

A flying shear usually comprises two rotating drums each equipped with aknife, and it is used for cutting either hot or cold steel strips. Thusa flying shear may follow a hot strip mill, a cold strip mill, or aprocessing line. The following description is based on the use of aflying shear in connection with a hot strip mill.

A hot strip mill comprises a series of mill stands. and the flying shearis placed immediately following the last stand, which is known as thefinishing stand.

One of the requirements of a flying shear is the cropping of the frontend of the strip at a predetermined distance from the end and thenshearing the strip to length. These lengths may vary from 15 to 30 feetor more. In order to crop the front end of the strip, the shear mustaccelerate very rapidly and also very accurately in order to make aconsistent front end crop at a predetermined distance from the front endof the strip.

After cropping the-front end. the shear runs at a constant speed, andmakes a length of cut dependent upon the relative speeds of the shearknives and the strip. Thus for a given strip speed. a definite speed ofthe shear knives will effect a deflnite length of cut. If the shearspeeds up, the cut will be shorter and if the speed of the sheardecreases, the cut will be longer.

Heretofore, flying shears have generally used an adjustable hydraulicgear for connecting the flying shear to the last stand of the mill.Although this type of mechanical tie between the mill and the shear hasgiven good results, it has also left much to be desired from the pointof view of accuracy and flexibility.

Accordingly, a further object of this invention is the provision ofmeans for eliminating the mechanical gear tie between the shear and thelast stand of the mill. and to provide in its stead a highly accurate.purely electrical tie.

The accuracies required are of the order of three-tenths of an inch in a30-foot length out. In other words. the accuracy desired isapproximately one-tenth of one per cent. An accuracy of one per cent isof course not required in all cases-but in general this figure is of theorder of theaccuravy that is usually required and it illustrates thehigh degree of accura y required in systerns of this character.

In order to have a range of 2:1 in length of cut.

it should be possible to adjust the flying shear over a 2:1 speed rangefor any particular strip speed, and then accurately to maintain thisspeed relationship. In addition, the delivery or strip speed may varyover a range of 2:1 and the flying shear should accurately follow anychanges in the strip speed.

Another operation of a flying shear is the cropping of either end orboth of the strip without intermediate cutting to length. In croppingthe front end. the shear is started so that it will out after a few feetof the end of the strip have passed through the shear. The shear mustthen be decelerated very rapidly to prevent making a second out. Incropping the tail end, the shear must accelerate to synchronous speed ofthe strip in less than the length of one cut, or otherwise a prematurecut would be made.

Thus it is obvious that for shearing, extremely accurate speed controlis required and for cropping, very rapid acceleration and decelerationare required. Likewise, to crop uniformly a predetermined distance fromthe end of the strip, the acceleration of the shear must be veryuniform.

For a better and more complete understanding of the invention, referenceshould now be had to the following specification and to the accompanyingdrawing, the single figure of which is a simple, diagrammaticalillustration of an embodiment of the invention.

Referring now to the drawing, a length of material l0, such for exampleas a strip of hot steel, is delivered from the finishing rolls H of thelast stand of a rolling mill and is passed between the upper an lowerdrums l2 and I 3, respectively, of a flying shear. The finishing rollsll of a rolling mill are illustrated as being driven by a direct currentmotor l4, although they might be driven by any other suitable drivingmeans. The shear drums l2 and I3 are illustrated as being driven bymeans of a direct current electric motor I! which is supplied from anadjustable voltage generator l8, which. in turn, is driven by suitablemeans illustrated as an alternating current induction motor I! suppliedfrom a suitable source of power represented by the three supply linesl3. Frequently in practice, flying shears for hot strips are driven bytwo motors in parallel in order to reduce the inertia and thereby toobtain the necessary accelerating torque. However, for the purposes ofsimplicity, the driving means are herein illustrated as the singledirect current motor it.

It will be noted that the flying shear is indicated in its initialpredetermined position ready to start a cropping operation. With theknives in this position, the maximum time is available for acceleratingthe flying shear before the cut is made.

Geared to the roll driving motor I are a small stand pilot generator I8and a small stand pilot tachometer generator 20. These generatorsproduce control voltages which are proportional to the speed of themotor I4 and therefore proportional to the speed of the strip in. Thesevoltages are used in the control system as reference speed voltages formatching the speed 0. the motor It with the speed of the motor ll. Theterm "matching" as used herein is to be understood as maintaining apredetermined speed relationship between the two motors.

A pilot tachometer generator 2i is geared to the shear motor I5 so as tobe driven thereby and to produce a voltage proportional to the speedthereof. The voltage of the shear pilot tachometer generator 2| is usedin the control system to'compare the speed of the motor is with thespeed of the motor H. A limit switch 22 is also driven by the shearmotor l5. This limit switch make". one revolution for each cut of theshear blades.

The shear motor supply generator I6 is provided with a field winding l6.and the excitation for this winding is supplied from "a suitabledynamoelectric machine which preferably is an armature reaction exciteddynamoelectric machine 23.

The dynamoelectric machine 23 is driven by any suitable means such as analternating current induction motor (not shown) at a speed which ispreferably substantially constant. The dynamoelectric machine' 23 hastwo sets of brushes. One set of brushes 23a is connected to the externalload which in this case is the separately excited field winding ills ofthe supply generator IS. The other set of brushes 23s isshort-circuited. The axis of the flux which is produced by the currentflowing in the shortcircuited armature conductors is referred to as theshort-circuit axis, and the axis which is displaced electric degreesfrom the shortcircuit axis is referred to as the control axis. If it beassumed that the axis of brushes 23b coincides with the short-circuitflux axis, then the load brushes 23. are on the control axis. The netflux along the control axis is produced by two opposing control fieldwindings 23 and 23. a series compensating fleld winding 23c, and thearmature reaction of the load current It is to be noted that the fieldwinding 23.1 is connected to be energized by the voltage drop across theline resistor 24 which is connected in series relationship between thesupply generator l6 and the shear motor. In other words, the controlfield winding 23.1 is energized in accordance with the current exchangedbetween the supply generator and the shear motor. The flux along theshort-circuit axis of the dynamoelectric machine 23 is produced by thearmature reaction of the short-circuit current. If desired, aportion ofthis flux may be produced by a shunt field winding (not shown). Thshort-circuit axis flux generates the voltage which appears across theload brushes 23., and the control axis fiux produces the voltage whichappears across the short-circuited brushes 23s and causes theshort-circuit current to flow.

The operation of the machine 23 will be readily understood byconsidering the operation without the second control field winding 23d.and the compensating field winding 23:, and then subsequentlyconsidering the effect of these fieds on the operation.

. Voltage may be applied to the voltage control field winding 23 andcurrent begins to build up in this field winding. Immediately there is gnerated a voltage in the short circuit path which circulates a largecurrent in the short circuit.

Since the resistance of the short circuit is so low as to be almostnegligible, the control field winding 23 is only required to produce avery small flux in order to produce alarge current in the short circuitand a correspondingly large armature flux. Furthermore, since the fluxof the control field winding need only build up to such a low value, andsince the resistance and reactsnce of the short-circuited armature pathare very low, full load current in the short c rcuit will be obtained inan excepiionnlly short Sta ss i iflsldt i i l t i 2. 9 t 's s it h ead.team-int pas es, t e. a a e ir action onh he -'control..,argl s can. besuostan- 'not ametes- ;sesame;m y. th

use. of, this {.series "compensatin'g ffield. windduce enough field fiuxand voltage to overcome theresistanceu drop in the short circuitbandaccordingly,.. the tinie constant orrate of response bei edcnt in' th fsc nslth'e two sites, .field windings flsmnd Zita-,oppose each other.

The control field jt is arranged to be connected to be responsivetc thedifference of a voltage of the pilot genetflWL-Jfi, and avoltage whichis appr ximately.proportional-to thespeed of the shear motor., l,,Eield-windlng-23 causeathe machine 2 3; t0 increase thevoltage on thegenerator lii, and thefleldwinding 23d. which is excited by.the,armaturecurrent ot shear motorsib causes the ,machine ,23. to.,tend to reducethe voltage 01. the machine 23 -and generator I8. 1

An, anti-hunting field 23M1S781S0, provided 0 the controlgaxis-ofqthemachine 23...;1It is energized from the -secondary .0! a transformer 25whose,primary, winding is connected across the:

armature load brushes 23L..- t

, A .constant-source; ofvoltage. issupplied to the buses, 26 ;from', asuitable source such for exam- -ple .as-an exciter. (not shown). a

The. basic operations -'-'f or-,controlling aecelera- .tion,deceleration-e :and. speed matching ,of .the ,motors llnandtiiiwareaccomplished by. utilizing thedynamoelectric machine; 23 :to regulatethe voltage applied-to: the :shear/ motor' but with recurrent-limit. ,-1

.To;this-; end. a: potentiometer rheostat :21- is connected across the,:armature". terminals of the shear motor, l 5 in suchza manner that thevoltage betweemthenegative terminal I: of the shear motor hand thecontact arm 11: is proportional to. thesidesired length; of'icut :Inother words, vwith the. contactermat;-the left-hand terminaloiethe-mheostat. :21 1 the shear motor supply generator l8-,would-:haveto go toimaximumavoltege :in iorder zto -match the :voltagerot .thestand :pilot generator: i9; iThis=would bertheconditionl for the-shortest: cut. ::On- 'the otherfi hand. with-the icontactuarm't2:1a;on:. the. right hand terminal: of

the rheostat: ZL-wthewhear generator would only .havato' go tota'iveryconsiderably lower voltage:

inorderto match the stand pilot generator voltagew' For. a'2i1' range inlength of cut,"it :wo1rld -be necessary for:"this rheostat toprovid a2:1 range'obvoltag'e'. -='-1he rh'eost'at-fl: is'hereinafter 'referred'tor: as thelength ot cut rheostatxr In sserieswith th'e length of cutrheostat is a rheostat 28 'here1nafter referred to' as the "rolldiameter compensating rheostat. Rolls of various diameters areused-on'the finishing stand andthis-oi course ca'use's the strip-speedto vary for: the same stand pilot genera-tonvoltage. :Accordingly, whenrolls of'a diiferent=diameterra'=correctioh4must'be 6 vacuum tubeamplifier 29 isiused the input oi which ladepend'ent upon thediilerencein voltage betweent he stand pilot tachcmeter generator 20 rand; the shear pilot tachometer generator 2|. In this vernier control. alength of cut rheostat 30 ,,and a rolldiameter compensating rheostat 3|are employed, Themovable c0ntacts-,30-.\:and 3|; of .these rheostats.are mechanically coupled to the movable contact arms 21 and 28 'of thecorre- .sponding rheostats in the coarse control as indica ted in;tl;e.drawing by the dot-tedzline connections. IFhe lengthof cut rheostat 30is calibrated ,so as.,-,to--give. the exact speed required for the.;iength,oi cut desired. Since the' direct'current vacuum ,tubes of; theampl-ifierylfllare one-way current devices, or rather rectifiers,:theamplifier ,liiwill: not pass'current as long as the shear pilottachometer-generator 2| is below the desired .speedv- In other.-.-words. the potential drop be- :tweemthe negative terminalpofcxthe standpilot tachometer: enerator 20 and theicontact arm of athe length of cut:rheostat. is greater than the potential drop across the shear pilottachometer generator whenthe shear is .below the desired speedt--'Iheresult of this isto tendPto force current through :the tubeamplifier;- in the wrong passesthroughthe tube amplifier. 1

,The length 0! cut rheostat 21 in the coarse control is so calibratedthat it tends to cause the shear-motor ii .to run at a speed that issubstantially 10 per cent greater than the desired speed.

A control potentiometer 32 is connected across thesource 26. Thispotentiometer is provided with a plurality of voltage-dividingtaps toprovide -.various controlling functions. t'tA line contactor 33illustrated as an electromagnetic switching device under the control ora-start switching device 34 and a stop switchingdevice 35 serves tocomplete and to interrupt the connections between the supply generator18 and the shear motor 15. Y -In actual practice an accurate timingcontrol is'provided' for closing a contact in parallel with thecontacts'oi the switch 34 for initiating the croppingoperation and alsoto open contacts in series-with the stop control-switchingdevice 35--to. initiate dc'celerat'on after-the completion ofthe-croppingoperation. However, since'the timing control-utilized forthispurpose constitutes .no-part of the present invention. the startcon- Htrolt'switching device 34 and the stop control switching device 35are illustrated simply as man-- -ually"0peratcd'-'type push buttonswitches. uwA'zswit'ching device 36 and an electromagnetic:

:c'o'ntactor 1311.: controlled thereby control the operation of cuttingto length. More specifica ly. :co'ntactor 31. controls the cutting inand cuttin ciit -oi the'length-of cut rheostat-Z'I and also the-cuttin'g in-and cutting out of the vacuum tilbt --atnplifier29..ErL-With the foregoing understand ng of the ci0- mentsand'their'organizatirn. the operation of -the'comp'lete system itselfwill readily be under steod from the fol owing dctailcd'description: Inthefollowingdescription,=it is assumed that tilt finishingstand motor-I4 -*is energized and is dr'ivingthe'finishing rolls-|l so that thefinished "strip'is' 'being"delivered to the flying shear drums 12; [3 ata. substantially coristantspeed.

ating coil of the length of out control contactor 31 so that when theswitching device 36 is subsequently closed, contactor 31 is energizedand is picked up to open its normally closed contacts 3k and to closeits normally open contacts 31!: and 31c. In the closed position ofcontactor 39, normally closed contacts 38a are opened to interrupt thedischarge circuit for the field winding I9 oi the pilot generator l9,and normally open contacts 38:: are closed so as to connect the voltagecontrol field winding 23 of the dynamoelectric machine 23 to the lengthof cut rheostat 21 and to the armature of the pilot stand generator l9.As a result, a control voltage is applied to the voltage control fieldwinding 23c which is equal to the difierence of the voltage of the pilotgenerator] 9 and the voltage between the movable contact arm 21a of therheostat 21 and the negative terminal I5 of the shear motor IS. Thelength of cut rheostat 21 has of course been previously preset for thedesired length of cut and as previously mentioned, rheostat 21 iscalibrated so that when preset for a speed corresponding to a desiredlength of cut, the coarse control actually tends to accelerate the shearmo or to a speed substantially per. cent greater than the desired.

speed. Simultaneously, normally closed contacts 381 are opened therebyto disconnect the voltage control field winding 23 from thepotentiometer resistor 32. Prior to the opening of contacts 38: thevoltage between the points 32a and 32s of the potentiometer resistor wasapplied across the contacts38ethereby'causing current to flow in thecontrol field winding 23; and as a result, the dynamcelectric machine 23energizes the field winding i6 so that the supply generator I6 wasgenerating a very substantial voltage when con-' tacts 38 were closed toconnect the voltage control field winding 230 to the stand pilotgenerator l9 and the length of cut rheostat 21. Thus at the instant ofclosing of the line'contactor 33, a voltage is available on the'supplygenerator Iii-which will give the maximum desired accelerating currentfor the shear motor immediately: Contacts 38g, in closing, connect thefield winding I9 of the stand pilot generator I9 tn the point 320, 32aof the potentiometer 32 with the reactor 39 included in seriesrelationship in the-circuit. Thus excitation is applied to the standpilot'generator I9. This excitation, however, does not build upimmediately because the reactor 39 limits the-rate of rise of current inthe field winding [9a. As the excitation builds up, the. stand pilotgenerator voltage builds up applying an excitation to the voltagecontrol field winding 23 oi dynamoelectric machine 23 in such adirection that the voltage at the brushes 23a rises. This increasingvoltage in turn raises the excitation of the shunt field winding i6. ofthe supply generator I5 and thereby increases the voltage applied to theshear motor [5. This process continues until the voltage between thenegative terminal 153 of the shear motor I5 and 'vernierspeed control.

'l9b of the stand pilot generator l9.-

the contact arm 21. oi the lengthoi cut rheostat 2| nearly equals thevoltage 01 the stand pilot generator l9. It the voltage between theterminal l5; and the contact arm 21. became exactly Qequal to thevoltage or the stand pilot generator 19, the excitation of the controlfield winding 23 would become zero and likewise the voltage of thegenerator l6 would become zero. However,

the acceleration of the shear motor l5 does continue until abalanced'conditioniiis established in which the net excitation onthecontrol field axis of the dynamoelectric machine 23 -is justsuflicient to cause the motor i5 to operatecontinuously at the speeddetermined by the setting of rheostat 21. v

If during this acceleration, theiarmature current of the shear motor l5tends to become excessive, the current limit control field winding 23awill-be energized from the line resistor 24 in series-with the shearmotorarmature in such a manner as to oppose the voltage control fieldwhich is causing the acceleration. Thus a current limit is imposedduring acceleration.

If-it were not for the vernier control, the shear motor would accelerateto a speed of 10 per cent faster than the desired speed, because or thecalibration of the length of cut rheostat 21, as mentioned in theforegoing. While the shear motor I5 is being accelerated to the desiredspeed, the voltage drop between the negative terminal 20. oi the standpilot tachometer generator 29 and the contact arm 30. of the length ofcut rheostat is greater than'lthe :voltage drop across the shear pilottachometer generator 2| and tries to force current through the tubeamplifier 29 in the wrong direction. Consequently, during theacceleration no current passes through the tube amplifier. However, theinstant that the voltage of the shear pilot tachometer generator 2|becomes-the greater, the tube amplifier will pass current and the outputof this amplifier will excite the field winding The connections betweenthe tube amplifier 29 and the field winding I91, are such that theexcitation of the field winding [91, is differential with respect to theexcitation of field winding 19., with the result that the voltage of thepilot'generator i9 is reduced. The lower the standj-pilot generatorConsequently, when pilot generator l9 actually is recalibrated in such amanner as to hold the exact speed desired as determined by the length orcut rheostat 30 of the fine control and the'shear-pilot tachometergenerator 2|. The tube amplifier 29 has a high degree oi. sensitivity.For example, the sensitivity .is preferably such that a variation of 0.1volt on the input will cause iull changerin the output. The number ofturns of the auxiliary field winding I9 is such that full change; in theoutput of the tube amplifier 29 will produce a change in the speed ofthe shear motor l5 of a substantial amount,,e. g. 15 to 20 per cent."Thus the pilot tachometer generators 20 and -2l together with the tubeamplifier'29 constitute 'a' highly accurate The length of cut rheostat30 in the vernier control actually determines the length of cut to bemaintained. and it is therefore not necessary to calibrate accuratelythe length of cut rheostat 21 of the coarse control as long as it tendsto cause the shear motor I5 to run at a speed higher than the actualdesired speed. Sincethe tube amplifier 23 is preferably a simpleconventional two-stage amplifier oi-the high vacuum or'hard" tubevariety, it-car'ibe arranged so that'it will operate with'a zerobias, 1. e., it can be-arranged so as to require practically no currentfrom the pilot tachometer generators. This is very important since theoperation is dependent upon voltage idio'ision and is therefore notsubiect to errors owing to changes in theresistanceotthe potentiometer30. The roll diameter compensation rheostat 3! does atfact the voltage,but since this is a small portion or rather percentage of the length'oic'utrheostat 30, such inaccuracies will be insignificant.

It is usually found that highly sensitive speed regulating systems suchas the tube amplifier verniercontrol system disclosed herein are sub-Jectto hunting. Accordingly, the tube amplifier 29 is provided with ananti-hunt transformer, the primary winding oi! which is connected acrossthe line resistor 23 and the secondary winding of which is connected bymeans or conductors 4| to the tube amplifier 29.

.To stop the shear, the push button switch 35 is depressed to openitsnormally closed contacts.

As pointed out in the foregoing, when automatic timing control is; used,contacts in series relationship with the switch 35 will be opened by theautomatic timingcontrol. Openingoi these contacts interrupts theenergizing circuit of the start contactor 33. In response todeenergization, contactor 38 opens its normally'open contacts 38b, 38s,-38s, and 38g and closesfits normally closed contacts 384 and 38:;Contacts 33 in opening disconnect the field winding I3. 01' the standpilot generator I! from the potentiometer 32, and contacts 38s inclosing complete a discharge circuit age is derived from thepotentiometer132 through a Jumper N from the right-hand contact 33. tothe tap 32.61 the potentiometer and from the lei thand contact 38.through normally 'closed contacts 33: and jumper 45, normally closedcontacts "46. of the-forward Jog contactor-'38" and converse thedirection of rotation of the shear'motor II. The reverse speed of theshear motor is defor this field winding through the resistor 42.

As a result, the 'standpilot generator voltage decays and the rate ofdecay is dependent upon the resistance oi'the discharge circuit. Thus,as the stand pilot generator voltage decreases below the voltagedropirom thenegative terminal l5. 0! the shear motor to thecontact arm. 21..orthe length of cut rheostat 21, the diiierencebetween these, twovoltagesreverses' the energization of the voltage control field .winding23c, and causes the output'voltage oi-the dynamo-electric machine 23 andthe voltage of the supply generator it to decay. Ii, duringdeceleration, thepump back current becomes excessivein the armature ofthe shear motor IS, the currentin the current limit' control fieldwinding 234 which --has reversed, owing'io the reversal of direction ofcurrent flow through the line resistor}! is therefore injsiich adirection as to oppose theexcltation eithevoltage controlfieldwindlng23e and thereby limits the pump back current to apredetermined value. The allowable peak current during acceleration or'deceleration is controlled by the current limit adjusting rheostat 33.The instantthatthe'speedof the shear'motor ifi' decreasesbelowthe.preset .desired value, no ourrentis'passed through the'tube amplifier 2aand the vernier speed tie system is entirely inactive iromthis' pointenduring theremainder oi the deceleration. v

Contacts m in opening do not interrupt the energizing circuit of thelinecontactor 33 which remains sealed in through the contacts of thelimit switch 22. Even though the voltage pf, the standpilot generator [8decreases to' zero, a voltage is imposed across the contacts 38.'whichis opposite in polarity with respect to the voltage of pilot generator[9 when generating. This volttermined by the magnitude of thereversevoltage derived from the potentiometerrheostat 32. When the imlveson theupper and .lower drums i2 and l3-oi tlie; fiying shear reach the desiredstopping position, the contacts of the limit switch 22 open andinterrupt the sealing-in circuit of the line contactor '33, whichin-r'esponse to demergization' opens its normally openg contacts 33-.33b, 33, and 33 and closes its normally closed contacts33s. 1

Contacts 33s in opening remove the reverse polarity voltage applied tothe fieldwinding 23 01 dynamo electric .machine 23 and contacts 33. inclosing complete a circuit from the left-hand contact 38e to the tap 32bof the potentiometer so that a .voltage of thesame polarity as thevoltage of pilot generator is is applied to the field winding 23s. Themagnitude of this voltage is determined by the position of the movablecontacts 32: on the.potentiometer. As afresuit, the pilot generator 23builds up and maintains a substantial voltage" on thesupply generator.It so that when the line contactor 33 closes contacts 331; at thebeginning of the next shearing operation, a voltage is available-on thesupply generator I 6 to provide the'desired accelerating current, forthe shear motor l5 immediately;

' During, acceleration, the. armature ,current oi the-shear motor itmaybe three oi, four times normal current, and as synchronousspee'd isappreached, the supply. generator voltage leads the speed of the shearmotor: l5 by a: considerable moving the contact 2 to thecorrect-position on the line ,resistor 24, so that the length of cutrheostat-21 insteadoi being connected directly across the terminalsoithe shear motor i5 is connectedlacross thearrnature ofthe motor i5 and aportion oithe line resistor 24. Theiurther-the contact 24. is movedtoward the positive terminal oi the supply generator IS, the less-jwillbe the tendencyior the voltage of the supply generator to lead the speedoi the shear motor l5, and

-a.slow tapering-oi! of the speedas the shear motorapproachessynchroncus speed is thereby obtained. In other v words, thedynamoelectrlc machine 23 receives the signal earlier. than 0therwisebefore the desired voltage of the supply generator l8 and the speed ofthe shear have been The accelerating current then decays and the voltageacross the shear motor l5 becomes more nearly equal to that acrossthegenerator Hi.

In cropping either the front end or the tail end of the strip, or both01 them, the accurate speed sary merely to accelerate to a speedslightly faster I I 2,998,877. tie between the shearand the finishingstand or the'.' rolling mill"isnot requi red and it is ,neoestends toaccelerate said -first' motorgtof-a: speed substantially, higher thanthe: desired ,,value, and

. .-'means responsiveto "the desiredij'speed relationthan the speed orthe stripand thendecelerate and stop the shear in position; In this casethe switch 3 6 is opened, which deenergizes he shearcontactor 31. Thisdisconnects thetube .a'mpli- "nerza so that the vernier ,spcedfcontrolis inactive.*The power control oi acceleration andde celerat ion isunderthetcontr'ol of .the dynamoelectric machine 23 as described in the,foregoingp Although in accordance with the provisions of the patentstatutes thisinvention is described as embodied in concrete form and.the principle thereof has; been explained, togetherjwiththe best' modein whichit'is now contemplated applying that principle, it will beunderstood that the apparatus and connections'snown' and described aremerely illustrativeand that the invention is not limited thereto. sincealterations and'modifications will readily suggest themselves to personsskilled in the art without departing from the true spirit of thisinvention or from the scope of the annexed claims.

What we claim as new and desire ,to secure by Letters Patent of theUnited State'sisi 1. Acontrol system comprising in combination anelectric motor, means for controlling the excitation ofsaicrmotorcomprising a dynamoelectric machine having a controlfield'axis, meansfor producing a control voltage comprising a generator having a fieldwinding, 9. source voltage, a reactance device and means for connectingsaid field winding to said source in series relationship with saidreactance thereby to control the rate of rise of the voltage of saidgenerator,

means for producing a voltage'proportional to the speed of saidmotor.'a'nd.means responsive l chine holds the speed of; said; its

stant at the desired value.

afirst 'electric motorg means3-; v .voltage approximatelyproportional;to the speed f ship=of said motors for varying:\the:;'voltage of said'generator so that; said :dyhar'rioelectric mamotor con- 1Agcontrolsystem comprisin nigmnibi mon, producing a of said motor, meansi or controllingthe speed of said motor comprising a dynamoelectricmachine provided with a control ;fie 1d': axisia second'electosaid'voltages for producing a-net' excitation along'said control fieldaxis thereby to accelerate said motor to" aspe'e'd having apredetermined relationship to the voltage of said generator.

2. -A'control system comprising incombination an electric motor, means,for controlling the speed of said motor comprising a dynamoelectricmachine having a control field axis, means for producing areferencecontrolvoltage, meansfor producing a secondvoltage proportionalto the speed of said motor, means responsive to both said voltages forproducing a net excitation along said control axis thereby to cause saiddynamoelectric machine to hold the speed of said motor i constant at avalue'having a predetermined relationship to-said control'voltage, meansfor varying said'second voltage so. that 'said dynamoelectric machinetends to hold the speed: of .said motor at a value substantiallyIgreater than the desired speed valuepand means responsiveto the desiredspeed of said motoriorvarying said control voltage so thatsaid'dynamoelectric machine holds the speed of said motor at thedesiredvalue.

3. A control system comprising in combination,

a first electric motor, means for .'controlling the speed of said motorcomprising a dynomo'electricv machinehaving "a control field axis, asecond electric motor and a pilot generator driventherebyfor producing areferencefcontrol voltage proportional tothe speed ofsaid"second*rnotor, means -for producing a [voltage "approximatelyproportional to the speed oivsaid first motor,

means'responsive to said voltages for produ'cin'ga tric,. motor and a'pilot generator-i drivem thereby .forproducing areference controlyoltage propor. tipnai to the speed 0i;said second-,motor, means vresponsive.to saidvoltages; iorproducing anet excitation alongsaidcontrol axis to :cause said dynamoelectric; machine to-,.acceleratesaid first motor, means {or varying said firs t vo ltage so that saiddynamoelectric machine tendstof,accelerate said ,firstmotor to aspeedsubstantially higher than the desired speed,-i =-a. tachometergenerator driven by said firstgrnotor a second tachometer; generatordriven by gsaid. second motor, andfmeans responsiveto thelyoltages ofsaid tachometer generator iondecreasing the excitation of said pilot;generatora. ;'predetermined amount when the' difierence -in speeds ofsaid motors is' aIpredetermined varr'ionnt thereby to cause. saiddynamoelectric machine to maintain the desired speed relationship oisaidm'otors'.

5. A control system comprising in combination,

' a firstelectric motor, a second'electric motor,

means ior' producing a vbltagejfapproximately proportional some speed ofsamj'arst motor, a

pilot generator having mainfand auxiliary field windings and driven 'bysaid secondelectric motor {or producing a reference control voltageproportional to the apeed of said second electric motor, means forvaryingathe speed of said first motor comprising a.dynamo'electricmachine having a control fieldfaxis, means responsive to,the diflerence of said yoltagesi iorf producing a vnet excitationalonglsaid control axis thereby to the excitation of said pilotgenerator is reduced when' the speeds ofsaid motorsfare inpredeterminedrelationship with each [other thereby to cause saiddynamoelectric machine to,maintain a ji gedetermined speedrelationshipbetween said 'motors.

6. A control system comprising in combination. a first=electric motor, asecond-fielectflc motor, means for 'xna ntaining a predetermined speed'relationship between said fmoto rsicompri'sing a pilot generatordriven -by' said second motor for producing a control voltageproportional to the net excitation along said control'axis thereby tocause saiddynamoelectricmachine'to accelerate said first'motor, meansfor modifying said net excitation so that said dynamoelectric machine'speedl'of said second motor, said-fgenerator having main and auxiliaryfield windings, means for producing a voltage proportional-to the'speedof said first motor and an armature reaction excited dynamoeletricmachine responsive to said volt ages for controlling the speed of"said-first mo-- tor, means-for varying the-voltage supplied tosaidarmature reaction machine so that said machine tends to hold the speedof said first motor ata value substantially higher than the desiredvalue, 'a first tachometer generatordriven by said first motor, a secondtachometer generator driven by saidsecond' motor, an electricvalve'amplifier is recalibrated when the speeds of said motors.

are in the desired relationship and operates to maintain said desiredrelationship.

7. A control system comprising in combination, a first electric motor, apilot generator driven by said motor for producing a control voltageproportional to speed, said generator having a field winding, a secondelectric motor, means for producing a voltage proportional to the speedor said second motor. means for controlling the speed or said secondmotor comprising a dynamoelectric machine having a control field axis, astarting control device and means controlled thereby for energizing saidgenerator field winding, means responsive to said voltages forcontrolling-said dynamoelectric machine to accelerate said second motorto a speed having a predetermined relationship to the speed of saidfirst motor,'a stop control device and means controlled thereby fordeenergizing said generator field winding thereby to cause saiddynamoelectric machine to decelerate and .everse the direction ofrotation of said second motor, and limit switch means actuated by saidsecond motor for terminating the reverse operation of said second motor.

8. A control system comprising in combination, a first electric motor, asecond electric motor, means for producing a voltage proportional to thespeed of said first motor comprising a pilot generator driven by saidfirst motor and provided with a field winding, means for producing avoltage proportional to the speed of said second motor, means forcontrolling the speed of said second motor comprising'an armatureexcited dynamoelectric machine provided with a control field windingconnected to be responsive to the difference of said voltages, a startcontrol device and means responsive to actuation of said device forenergizing said generator field winding to efiect acceleration of saidsecond motor to a speed having a predetermined relationship to the speedoi'said first motor, a stop control device, means responsive.to.actuation of said stop control device ior deenergizing saidgenerator field to eiiect deceleration of said second motor and forapplying a voltage of reverse polarity to said con- Q a control fieldwinding connected to be responsive to the diiierence or saidvoltages, astart controlswitching device, means responsive to actuation oi saidswitching device energizing said ge'neratorfieid winding to efiectacceleration of said second motor, a stop control device, meansresponsive to-actil'ation of'said'fstop control device rorideenergizingsaidgenerator field winding to'efiect deceleration of said motor, andmeans trol field winding to effect reverse rotation of said secondmotor. and limit switch mechanism operated by said second motor forterminatin said reverse rotation. I

9. A control system comprising in combination, a first electric motor, apilot generator driven thereby for producing a control voltageproportional to its speed, said generator beingprovided with acontrolfield winding, a second electric motor, means for producing a voltageproportional to the speed of said second motor, means for controllingthe speed of said second motor comprising a dynamoelectric machineprovided with responsive to the'armature current of said second motorfor producing a componentof excitation opposing the excitation of; saidcontrol field winding thereby to limit the armature current of saidsecondimotor during acceleration and deceleration. i 4

10. Acontrol system comprising in combination,: a; first electric motor;a pilot generator driven by said motor for producing a'control voltageproportional to the speed of said motor, said generator being providedwith afield winding, an

' adjustable voltage generator, and a second electric motor suppliedtherefrom, means for pro ducing a control voltage proportional to thespeed oi said second motor, means for controlling the excitation of saidgenerator comprising a'dynamoelectric machine provided with a controlfield winding connected to be responsive to the difference of saidvoltages, a start control device and switching means controlled therebyfor connecting said pilot generator field to a source of supply therebyto accelerate said second motor, a stop control device, switching meansresponsive to operation or said stop control device for deenergizingsaid pilot generator field winding thereby to eilect deceleration ofsaid second motor, and means responsive to the stopping operation ofsaid second motor for applying a voltage to said control field windingthereby to cause said supply generator'to apply a voltage to saidsecondmotor to provide rapid acceleration of said second motor in response tothe next operation of said starting device.

11. A control system comprising in combina tion, a first electric motor,a second electric motor, coarse control means for maintaining apredetermined speed relaticnship between the speeds of said motorscomprising a pilot generator driven by said first motor for producing avoltage proportional to the speed thereof, means for producing a voltageproportional to the speed of said second mo *1 and a dynamoelectricmachine having a conti field winding connected to be energized by saidvoltages for controlling the speed of said second motor, a rhoostat forvarying the relative values of said voltages to control the relativespeeds of said motors, saitl rheostat being calibrated so as to causesaid dynamoelectric machine to maintain the speed of said elementdriving motor at a value substantially higher than the desired value,and fine control means comprising a pilot tachometer generator driven bysaid first motor, a second pilot tachometer generator driven by saidsecond motor, means responsive to a predetermined relationshi of saidtachometer generabor voltages for reducing the voltage of said firstmentioned pilot generator so that said dynamoelect'ric machine maintainsthe desired speed relationship between said motors, a rheostat forvarying the relationship of said tnchometer generator voltages therebyto adjust the relative speeds of said motors. and a second rheostat forfurther modifying said voltages.

MAR'I'INMA. EDWARDS. FRANCIS MOHLER.

CERTIFICATE OF CORRECTION.

Patent No. 2,298,877. October 15, 1911.2.

MARTIN A. EDWARDS, ET AL.

It is hereby certified that error appears in the printed specificationof the above numberedpatert requiring correction as follows: Page 1,sec.- ondcolumn, line 14.9, after "accuracy" insert --of one-tenth--;line 52, for "accuravy' read "accuracy"; page 5, first colmnn, line 55,for "58 read 58 and that the said Letters Patent should be read withthis correction therein that the same may conform to the record of thecase in the Patent Office.

Signed and sealed this Ehth day of November; A. D. 1%2.

Henry .Ven Arsdale, (Seal) Acting Commissioner of Patents.

CERTIFICATE OF CORRECTION.

Patent No. 2,298,877. I I October 15, 19LL2.

MARTIN A. EDWARDS, ET AL.

It is hereby certified that error appears in the printed specificationof the above nmnberedpatent requiring correction as follows: Page 1,sec.- ondcolumn, line 11.9, after @zccuracy" insert a of one-tenth--;line 52 for "accurayy" read --accuracy-.; page 5, first column, line55', for""58 read. 58 and that the said L's/star's Pgtenjl: should beread with this correction therein that the same may conform to therecord of the case in the Patent Office. 4

Signed and. sealed this a m day of- November, A. n.'19l+ 2.

Henry Yen Arsdale, (Seal) Acting Commissioner of Patents.

